In experiments number one and two, four chickens and three pigeons were not infected when subjected to skin inoculations containing parasites doses of 9.9 X 10*5 and 3.8 X 10*6 both from culture plus blood borne forms. In experiment number 6 the refractory state was not broken when two other chickens received every other day 5 intraperitoneal injections of 2.8 X 10*6 to 6.1 X 10*6 parasites or 5 intraperitoneal injections daily of the parasites from a culture tube suspended in saline plus blood borne forms from 3 infected young mice. The refractory state was not broken also in two chickens from experiment number 7, where dexamethasone was associated to the intraperitoneal parasites doses of 8.1 X 10*6 and 3.5 X 10*7, respectively. This drug was administered either on the first day (0.4 mg/8 doses/10 days) or three days prior to the inoculation (0.4 mg/10 doses/12 days). In these 11 birds the refractory state was proven following blood inoculation to newborn albino mice (54 negative inoculations in 301 mice), 52 negative xenodiagnosis tests (175 larvae and adults from Triatoma infestans and Panstrongylus megistus). The inoculations and the xenodiagnosis were performed between 1 and 96 hours and on the 10th, 20th and 30th days after the bird were inoculated. Therefore, besides the refractory condition it was alsoshown that the parasites do not enter the peripheral blood. Table 1. In the experiments number3, 4 and 5, a total of 10 chickens were inoculated in the skin with 1.8 X 10*6 to 3.6 X 10*6 parasites and viable forms were found in the site. In these areas punctions were made every half-anhour and the harvested material inoculated into newborn mice. From 17 inoculations performed within 8½ hours, 12 were positive, including the last one; and from 12 performed between 9 and 90 hours (the first four still made every half-and-hour), all were negative. On the other hand, from 10 xenodiagnosis made in the sites of inoculation between hal-and-hour and 9 hours after inoculation, two were positive; and all xenodiagnosis performed between 9 and 90 hours were negative. When 10 xenodiagnosis were repeated in 4 new chickens (experiment number 8) between half-an-hour and 10 hours only 2 were positive (7 and 8 hours). Table 2 . The xenodiagnosis under the experimental conditions of this work, as a laboratory diagnostic test, was inferior to the newborn mice inoculation. Then viable parasites were still present in the inoculated areas at least 8½ hours after inoculation. However, the number of parasites decreased progressively in the inoculated areas, 2 hours after inoculation, showing at the same time clear morphological alterations as well as many dead forms. Under direct examination none was seen after 5½ hours of inoculation. The inoculation cause an inflammatory picture of chicken skin that envolves both the adipose tissue and muscles leading to abscesses formation. Phagocytosis which is chiefly produced by heterophil leucocytestakes place...